SOS: Hydroboost/MC battle

[302BRONC]

Ok bronco brothers - this is not easy for me to reach out for help like this, but I have reached the end of my rope and need this thing stopping in order to make the Stampede. I thank you for your time in advance.

I've been fighting the new brake system on my 66 through 3 MC's so far.

Setup as it sits now:
77 Corvette disc/disc MC 1.125 bore- New -
Directly bolted to:
Astro Hydroboost, straight style - j/y used
Modified pedal rod to stock bronco pedal assembly
79 F250 dual piston front calipers - Reman -
79 Chev 3/4 ton rear single piston calipers - used (they were on the axle when I bought it)
All new hard lines - mostly 3/16" along frame rails with 1/4" on the front housing - bent by me
All new soft lines with very minimal amount of adapters (1 on front housing)
All four bleeders on calipers are pointed up
Front brakes hooked to front port and Rear brakes to rear port of MC (as i understand is the correct way for the Vette MC).

Synopsis of events and issues:
MC #1 - inadvertently ordered a 1970 Vette MC which is a manual style and the hydroboost rod isn't long enough, crap - back to the store.

MC #2 - 77 Vette - bench bled thoroughly (they hold a good amount of air). Bolted the MC up, double checked fittings throughout system and proceeded to bleed furthest caliper to closest. We could not get the rears to bleed much, just a very small amount of pressure at the bleeder screw. Unbolted calipers and inserted 2x4 to make sure bleeder is at 12 o'clock. The caliper would not even make a dent in the wood. So we decided to check flow and removed the sort caliper line and we were still finding small amount of fluid coming out of line when pedal is depressed. We checked the next line up all the way to the MC and they all had the same low flow. Low and behold, the MC is putting out a puny amount of fluid at the rear port. We hooked the system back up and we bled the fronts fine with appropriate amount of air escaping and then good fluid flow. With this we could lock up the front brakes enough to drag 42" tires and the rears were doing nothing. For giggles, we swapped ports on the MC Fronts on rear port, Rears on front port. And still could lock up the fronts, but do nothing on the rears...... odd. Decided we didn't trust the "NEW" MC and ordered another.
Couple things to note on pedal feel here:
1) Pedal is stopping roughly 1.5" from the floor (you can push through this to the end of the MC stroke)
2) It is not increasing pressure as you pump
3) When back bleeders are cracked, pedal will not release to the bottom of the stroke, but will with the fronts.
4) There is a small thud feeling in the pedal near the end of the stroke.

MC#3 - 77 Vette - Bench bled very thoroughly by tapping with mallet, holding at angles, etc. Bolted the MC up, double checked fittings and proceeded to bleed system furthest to closest. Rears are bleeding! Appropriate amount of air out of the rears and then good fluid pressure after the air is gone. At this point, i'm thinking we're golden. Start bleeding fronts and we find the same symptoms as the rears with MC#2! Only a very small amount of fluid is coming from the bleeders, pedal is not building pressure when pumped. We chased back the lines to the MC again and find again, a puny amount of fluid is being expelled from the Front port this time!! Ahh! Again, for giggles we swapped lines Front on rear port, Rears on front port. Rear brakes are definitely grabbing, but won't drag the tires.
Pedal Notes:
1) Pedal is still stopping roughly 1" from the floor (You can push through this to the end of the MC stroke)
2) It is not increasing pressure as you pump
3) When FRONT bleeders are cracked, pedal will not release to the bottom of the stroke, but will with the REARS.
4) Small thud feeling in the pedal near the end of the stroke IS GONE

I'm not thinking it is the Astro HB unit as the MC doesn't care what is pushing it. And it doesn't matter whether the truck is running or not, the same symptoms are arising.

I'm baffled why the brake issues have completely flipped 180 on me.

Is it that the 1.125" bore on the MC is so far off that it cannot handle the 6 total pistons on the truck?
Did I get another botched Master Cylinder?
Are my lines undersized?
Why is the pedal not acting normally when we pump it up (think Hanz and Franz) to build pressure to bleed?
Is there just an unreasonable amount of air in the system still?

Thank you guys again for any help.

 

[phldwsnoc1]

Wow. You win the Frankenbrake award. I feel qualified to answer a couple of your questions. First, line size is not your problem. Second, reman master cylinders are always suspect. Third, unless your lines are run in a descending configuration from source to wheel, you may need pressure or vacuum to clear the air. Best of luck. I admire your ambition!

 

[bronconut73]

How is the HB rod adjusted where it enters the MC? Could it not be adjusted out enough or too much?
Either would be an issue.

 

[OX1]

Is it that the 1.125" bore on the MC is so far off that it cannot handle the 6 total pistons on the truck?

No, I have almost exactly the same brake setup (calipers and MC bore, SVO MC for me) with a late 70's linc HB (on a 79 bronco) and it works fine. I'd guess that linc HB has smaller bore than your astro.

Did I get another botched Master Cylinder?

Is engine running with HB creating "boost" when you bleed?
I've had that problem with very little flow with some vac brake setups.
Never figured out why, but once engine was running and line pressure was up, they bled fine (almost too good sometimes, have to work pedal slow or fluid just pisses out everywhere).

Are my lines undersized?

No......

Why is the pedal not acting normally when we pump it up (think Hanz and Franz) to build pressure to bleed?
Is there just an unreasonable amount of air in the system still?

Guessing yes in the rear, so you are just pumping against it.
When you release, pressured air just sends fluid backwards
and it will never pump up.

I would also suggest setting booster rod out (if adjustable) so that it touches MC piston slightly before you are able to bolt MC up (like 1/32-1/16). This will help ensure you have all your travel and then some to bleed. Can always re-adjust later and back it down. I have VERY slight pressure on MC piston from rod in my rock crawler all the time, as I like that brake feel to be "right there". Does create very slight drag all the time, but my speeds are slow and it is trailer queen.

 

[Hozr]

I'm not positive you have the front/rear lines correct at the M/C. Generally the larger reservoir will connect to the front brakes.

Was the P/S system bled thoroughly?

What happens if you activate the brakes without the hydroboost? (manually push the m/c while connected to system)

The weird pedal feel makes me think the hydroboost is bad internally.

 

[302BRONC]

Wow. You win the Frankenbrake award. I feel qualified to answer a couple of your questions. First, line size is not your problem. Second, reman master cylinders are always suspect. Third, unless your lines are run in a descending configuration from source to wheel, you may need pressure or vacuum to clear the air. Best of luck. I admire your ambition!

Thanks - yes, I've always had a skill for making things harder than they have to be. Lines are descending from MC to the calipers minus a couple loops on the drop lines for articulation.

I'll be trying to repeat this all with the hydroboost active next.

 

[302BRONC]

Is it that the 1.125" bore on the MC is so far off that it cannot handle the 6 total pistons on the truck?

No, I have almost exactly the same brake setup (calipers and MC bore, SVO MC for me) with a late 70's linc HB (on a 79 bronco) and it works fine. I'd guess that linc HB has smaller bore than your astro.

Check. cross that off the list.

Did I get another botched Master Cylinder?

Is engine running with HB creating "boost" when you bleed?
I've had that problem with very little flow with some vac brake setups.
Never figured out why, but once engine was running and line pressure was up, they bled fine (almost too good sometimes, have to work pedal slow or fluid just pisses out everywhere).

We have tried both ways, but majority of the bleeding effort is without boost and strong-legging the pedal.
I'll try to repeat the entire process with the HB active to see what that does.

Are my lines undersized?

No......

Groovy.

Why is the pedal not acting normally when we pump it up (think Hanz and Franz) to build pressure to bleed?
Is there just an unreasonable amount of air in the system still?

Guessing yes in the rear, so you are just pumping against it.
When you release, pressured air just sends fluid backwards
and it will never pump up.

I'll continue bleeding. I think i'll open bleeders to gravity them for awhile and go from there.

I would also suggest setting booster rod out (if adjustable) so that it touches MC piston slightly before you are able to bolt MC up (like 1/32-1/16). This will help ensure you have all your travel and then some to bleed. Can always re-adjust later and back it down. I have VERY slight pressure on MC piston from rod in my rock crawler all the time, as I like that brake feel to be "right there". Does create very slight drag all the time, but my speeds are slow and it is trailer queen.

Good suggestion on the booster rod. Right now it is sitting just shy of the MC piston when everything is at rest. I have measured stroke on the MC and it is definitely less than the stroke on the HB, assuming of course the stroke of the HB is equal when the MC is not on the truck (how i can measure it) vs when the MC is bolted up.

Thanks for your time!

 

[302BRONC]

I'm not positive you have the front/rear lines correct at the M/C. Generally the larger reservoir will connect to the front brakes.

The reservoirs are roughly the same size and once disassembled (don't tell Napa) the front plunger (for the front port) has the weaker spring indicating it will actuate first (presumably to grab front brakes first)

Was the P/S system bled thoroughly?

I do admittedly have some air in the P/S system yet. everything (steering box, hydro assist, hydroboost) works and the pump doesn't squeal, but I do get less and less minor bubbling every time I play with it. Do you think the root of the issue is the HB unit not being fully bled? It was my understanding that i have a direct connection to the pedal through the HB and the operation of the HB is irrelevant.

What happens if you activate the brakes without the hydroboost? (manually push the m/c while connected to system)

Most of the work i outlined above happened without the truck running and therefore no hydroboost. Again, because i understood that the MC doesn't care what is pushing it (vac booster, hydroboost, or manual) as long as it's pumping.

The weird pedal feel makes me think the hydroboost is bad internally.

I do have another HB unit sitting around.. if the next round of bleeding doesn't take care of it, I'll look into swapping the units.

One thing i did notice as far as condition of the HB unit is that the Nitrogen canister is loose in the casting. I think this is due to all of the pumping without the truck running and it not creating pressure in the canister to hold it tight against the casting. But, is this normal?

Thank you.

 

[toddz69]

One thing i did notice as far as condition of the HB unit is that the Nitrogen canister is loose in the casting. I think this is due to all of the pumping without the truck running and it not creating pressure in the canister to hold it tight against the casting. But, is this normal?

No, that means the accumulator is bad and needs to be replaced. I'm not sure if they are sold as a replacement item or not. I changed one once and was nervous during the whole process.

And yes, on the Corvette masters, the front port is front brakes. Rear port is rear brakes.

Todd Z.

 

[302BRONC]

No, that means the accumulator is bad and needs to be replaced. I'm not sure if they are sold as a replacement item or not. I changed one once and was nervous during the whole process.

Todd Z.

OK great to know, I'll swap it with the other unit I have to eliminate (hopefully) that issue and start over.

 

[Hozr]

Yeah, if the accumulator is loose it won't function properly.

 

[302BRONC]

Alright gents- new (used) HB unit on. Bled P/S system. The pedal drops to the floor when the truck is running and the HB unit appears to be applying the brakes. We bled the brakes with MS #3 and the new HB, seems to be acting more normal without truck running.

 

[wgamble]

I do not know hydroboost on one of my crawlers, we used a master cylinder from an f350 with an extra PSC can to make sure that there is enough fluid being pushed at all times. Do you have anything helping you push it?

 

[Hozr]

How do you have the HB plumbed?

Should be Pump to passenger side (fittings on top)

Driver side to P/S box

Sounds like there is still air in the system somewhere.

How did you bleed the HB system?


Important: Power steering fluid level must be maintained throughout bleed procedure.
Fill pump reservoir with fluid to minimum system level, FULL COLD level, or middle of hash mark on cap stick fluid level indicator.Important: With hydro-boost only, the oil level will appear falsely high if the hydro-boost accumulator is not fully charged. Do not apply the brake pedal with the engine OFF. This will discharge the hydro-boost accumulator.
If equipped with hydro-boost, fully charge the hydro-boost accumulator using the following procedure:2.1. Start the engine.
2.2. Firmly apply the brake pedal 10-15 times.
2.3. Turn the engine OFF.
Raise the vehicle until the front wheels are off the ground. Refer to Lifting and Jacking the Vehicle (http://gsi.xw.gm.com/si/cellHandler.do?cellId=47852&refDoc=487449&from=sm) in General Information.
Key on engine OFF, turn the steering wheel from stop to stop 12 times.Vehicles equipped with hydro-boost systems or longer length power steering hoses may require turns up to 15 to 20 stop to stops.
Verify power steering fluid level per operating specification. Refer to Checking and Adding Power Steering Fluid (http://gsi.xw.gm.com/si/cellHandler.do?cellId=54933&refDoc=487449&from=sm) .
Start the engine. Rotate steering wheel from left to right. Check for sign of cavitation or fluid aeration (pump noise/whining).
Verify the fluid level. Repeat the bleed procedure if necessary.

 

[302BRONC]

FIXED! Well- sort of.
After the new HB install, i actually pulled the old and new ones apart and only swapped the valving portion closest to the MC so i didnt have to mess with making a new pedal rod. Upon reinstall, i failed to place what i gather is the mechanism that returns the pedal correctly. Once installed correctly, the diving pedal issue was gone.

On to the MC#3 not giving me enough fluid, we grabbed a MC off of a 76ish el Camino that happened to not be needing brakes as the engine is in pieces. We flushed thoroughly, bench bled and bolted it up. Bled the rears and got plenty of air, then fluid. Moved to the fronts and we finally have air amd fluid coming out! after a couple more rounds of bleeding, we can kill the engine with the brakes!

So success, but frustrating. Why did the legendary 77-82 Corvette MC -2 of them- not want to play nice?

Thanks for the P/S bleeding info HOZR. Helped a ton.

 

[302BRONC]

And thanks to everyone else that commented. The suggestions and the subsequent reaults led us to try another (different) MC.

 

[Hozr]

Awesome! Not sure what the deal is with the Corvette M/C's. The bleeding sequence can be critical in some situations.